1. Field of the Invention
The present invention relates to a method of and an apparatus for determining the individual existence of a first group of particles contained in a fluid and a second group of particles in the same fluid but with characteristics different from the first particles. More specifically, the present invention relates to a method and an apparatus capable of distinguishing particles in a fluid that can be advantageously adopted as a method and an apparatus for, for example, distinguishing ion exchange resins used in the purification of water to produce highly pure water, or monitoring the state of such ion exchange resins during their regeneration.
2. Related Art
Ion exchange is performed to produce highly pure water which is required, in great amounts, by facilities such as thermal power plants, nuclear power plants and large, semiconductor-manufacturing plants. In general, an ion exchange operation conducted for this purpose comprises exchange and reduction which are repeatedly performed. A reduction includes backwashing, separation, regeneration, and washing, which take place in the mentioned order.
Such an ion exchange operation may be performed using a mixed-bed deionizer which is advantageous in that the desalted water is both highly pure and neutral and that silicates dissolved in the fluid can be completely removed. In this case, the conventional practice is such that a strongly acidic resin (positive ion exchange resin) and a strongly basic resin (negative ion exchange resin) are charged into a single ion exchange tank. When the resins are evenly mixed together with the aid of air, it is possible to obtain innumerable combinations of the positive ion exchange resin (hereinafter referred to as "cations") and the negative ion exchange resin (hereinafter referred to as "anions"), thereby enabling the production of highly pure desalted water. The two types of resin particles, which move according to Stokes' Law, differ from each other in specific gravity, and this difference results in different sedimentation velocities. Utilizing this fact, backwashing is effected to separate the two types of resins from each other. The separated resins are then regenerated.
An example of a mixed-bed deionizer is shown in FIG. 8. An ion exchange tank 10 contains anions and cations. A first solution 15 comprising sodium hydroxide (caustic soda) is used to regenerate anions, while a second solution 16 comprising hydrochloric acid or sulphuric acid is used to regenerate cations.
FIGS. 9 (a) through 9 (g) show an example of the operation of a mixed-bed deionizer. FIG. 9 (a) shows desalting, FIG. 9 (b) shows backwashing and separation, FIG. 9 (c) shows regeneration of anions, FIG. 9 (d) shows regeneration of cations, FIG. 9 (e) shows washing, FIG. 9 (f) shows mixing, and FIG. 9 (g) shows initialization. These processes are performed in the same order in which they are shown in FIGS. 9 (a) to (g), and cyclically so that an initialization process, in FIG. 9 (g), is followed by a subsequent desalting, in FIG. 9 (a).
In FIGS. 9 (a) to 9 (g), the hatching with lines declining rightward shows regions where anions exist, the hatching with lines declining leftward shows regions where cations exist, and the hatching with criss-cross lines show regions where anions and cations are mixed together.
When the above-described ion exchange operation is being performed, various valves through which water and necessary chemical solutions are introduced into the tank are manually operated to adjust flow rates, whereby the position of the boundaries between the water, the anions and the cations, being visually monitored, is brought to the correct position.
A drawback of a conventional ion exchange operation is that, because valves are manually operated while the state of the ion exchange resins in the tank is visually observed, the operation inevitably requires human labor.
To automatize an ion exchange operation, it is necessary to automatize the valve adjustment for determining flow rates as well as the detection of the state of ion exchange resins. Although the valve adjustment can be automatized with conventional techniques, it is difficult to automatize the detection of the ion exchange resin state with conventional optical sensors or video cameras. This is for the following reasons: particles of ion exchange resins vary in size; particles are distributed in water or solutions at different densities; and it is difficult, in general, to distinguish cations and anions from each other because, although they can be correctly distinguished when they are new, their distinguishing from each other becomes difficult as time passes after their first use because clad deposits on and adheres to the surface of the resin particles and thus changes the reflectivity at which light is reflected by the resin particle surfaces.